Although the concentration of gasses like sulfur dioxide and nitrogen dioxide remained within safe limits on the night of Diwali this year, the city noted higher air pollution although the noise pollution dipped as compared to last year. Of the 12 observatories, sulphur dioxide concentration increased at nine locations and decreased in three compared to 2011. Concentration of SO2 ranged between 13 and 63 µg/m3 (microgram per cubic meter of air) as compared to 4 and 40 µg/m during Diwali in 2011. SO2 levels have been found within the prescribed standard of 80 µg/m3 at all locations. The concentration of nitrogen dioxide in the air has also increased slightly, but has remained within the permissible limit. However a steep increase has been recorded in the presence of Particulate Matter (PM10) in the air. The normal range of the PM10 concentration in the air remains between 400 to 500 µg/m3, however on November 13 it was recorded at over 900 µg/m3 in some observatories. Observatories at Janakpuri, Shahdara and Shahazada Bagh recorded the highest concentration of Particulate Matters with 951, 928 and 820 µg/m3. The overall concentration of these matters has doubled as compared to the last year figures. However the CPCB feels that burning of crackers is not the only reason for the rise in Particulate Matter in air. “Besides fire crackers, the number of vehicles on the road is high on the day of Diwali. The smoke emitted from these vehicles also add on to the PM10 levels,” explained Gurnam Singh of the CPCB Air Laboratory. The high relative humidity content of the air, decrease in temperature and slow wind speed has added to the accumulation of the particulate matter in the air. “While the wind speed is good at the upper latitudes, it has slow close to the surface. Humidity level in the air remained high, resulting in the suspension of particular matter close to the surface,” airport Met director RK Jenamani said. The maximum relative humidity on Tuesday was recorded at 68 percent compared to 39 percent of the previous year.